1. Field of the Invention
The present invention relates to thin films an d particularly to a thin film which adheres directly to glass, ceramic, and other substrates and which has excellent wettability for solder.
2. Technical Background
Solderable thin films are employed in a variety of electrical components including photonic devices, optical fiber packaging, semi-conductor devices and flat panel displays. Typically, Multiple layers of films are employed, such as indium tin oxide (ITO), which has good adhesion to substrates such as silicone dioxide, aluminum oxide, and other ceramic materials. Such materials, which adhere well to glass and ceramic substrates, typically do not bond well with solder for providing electrical connections to such films. As a result, films such as ITO films must be covered with an outer solderable layer, such as copper, silver, gold, or platinum, so that electrical contacts can be made between devices mounted on a glass or ceramic substrate and external components.
Although copper and gold films are wettable to solder, they have poor adhesion to glass substrates. Silver film has reasonable adhesion and excellent solder wetting properties but suffers from electro-migration and, therefore, reliability. Aluminum and chromium films also adhere well to glass substrates but are not wettable to tin-lead solders. Thus, in the past, multi-layered coatings have been employed with a first layer having excellent adhesion properties deposited onto a substrate followed by one or more over layers of material with the last layer having excellent wettability to solder. Depositing such multiple film layers required heating at temperatures of from about 150xc2x0 C. to 250xc2x0 C. to assure adhesion. When, however, heating a substrate during such thin film coating, some devices can be destroyed or their properties adversely affected.
Another difficulty with some thin films results from the fact that the internal strain energy of a film is proportional to its thickness. In thicker multi-layer films, internal strain energy may exceed the adhesion to the film substrate causing a failure of the component using such a film. Also, multi-layer solderable coatings may have a relatively high contact resistance. Typically, adhesion enhancing layers, such as ITO, zinc oxide, and tin oxide, or highly resistive metal layers, such as chrome, Nichrome(copyright) and the like, have relatively poor conductivity and cannot achieve a good ohmic contact with the wettable over-layer of copper, gold, or other wettable material.
There exists, therefore, a need for a thin film coating which can be applied at a relatively low temperature, which has a low internal stress when applied, strong adhesion to a variety of substrates and excellent wetting properties for solders employed, such as tin-lead, and gold-tin, and a coating which is environmentally stable.
It has been discovered that an alloy of copper and gallium provides an excellent thin film material which adheres well to a variety of nonconductive substrates, such as glass, ceramic, and polymeric materials, and which can be applied at a relatively low temperature with minimal internal stress, such that the adhesion to the substrate resists displacement of the thin film from the substrate. Additionally, a copper-gallium alloy provides excellent wettability to solders such that electrical contacts to photonic and electrical components can be made. In a preferred embodiment of the invention, the copper-gallium alloy was from about 1 to about 40 percent gallium and from about 99 to about 60 percent copper on an atomic basis (i.e., percentage of atoms in the alloy) and was deposited to a thickness of from about 400 nanometers to about 3 microns at relatively low temperatures from about room temperature (20xc2x0 C.). In some applications, it may be desirable to deposit the coating at higher temperatures. The copper-gallium alloy can be deposited over a range of about 20xc2x0 C. to about 300xc2x0 C., if desired. The copper-gallium film is deposited utilizing conventional sputtering, electron beam, or other deposition equipment. The resulting thin film can be used, in addition to providing electrical contacts for photonic and electrical components, to provide a surface for solder sealing a component within a hermetically sealed container, frequently used with optical components in telecommunications.
The present invention, therefore, contemplates a method of depositing a copper-gallium alloy on an insulative substrate to provide a thin film layer. The invention further includes pholonic and electrical components which utilize a copper-gallium thin film layer on an insulative substrate for providing electrical contacts for receiving soldered connections. The present invention further contemplates the utilization of copper-gallium thin film on substrates for packaging components by sealing such components utilizing a solder interface as the seal and a combination of sealing and providing electrical contacts.
It is to be understood that the foregoing descriptions are exemplary of the invention only and are intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provided a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.